1. Field of Technology
The present application relates to the field of antennas. Specifically, it relates to interference suppression (IS) and direction-finding (DF) systems.
2. Related Art
Intentional and unintentional interference is a common problem in the field of wireless communications. Interfering signals often share the same frequency band (or channel within the band) as the desired signal. When the desired signal arrives along a reflected path or paths it too can behave like an interference signal. This is often referred to as multipath or coherent interference, which can lead to partial cancellation of the signal strength. This in turn can result in signal fade or dropout. Signals unrelated to the desired signal are referred to as incoherent interference. Incoherent interference can be either broadband or narrowband. Broadband interference is spread over a large fraction of or all of the bandwidth associated with the desired signal. This interference looks like noise to the system and will effectively reduce the signal-to-noise ratio (SNR) and can swamp the desired signal or at least reduce its quality. Narrowband interference occupies a smaller fraction of the signal band. Applying narrowband-filtering or narrowband-processing techniques to the received signal can sometimes mitigate the harmful effect of narrowband interference. In the case of digital communications, both coherent and incoherent interference can lead to unacceptable bit error rates, loss of signal lock, or a corruption of the information or message in the desired signal and hereinafter “interference” refers to both coherent and incoherent interference unless otherwise indicated.
Civilian and military navigation systems increasingly rely on the accuracy of information provided by the Global Positioning System (GPS). Moreover, since GPS receivers are now embedded in many different types of systems, this dependence goes far beyond navigation and guidance systems and extends into areas such as personal communication systems (PCS) and wireless internet access systems. In military systems, the loss of GPS signal lock could cause an otherwise successful mission to fail, endangering the lives of soldiers and noncombatants, and wasting valuable resources. The same is true in civilian applications. Therefore, it is imperative not only to protect the integrity of the GPS signal, but to locate and to eliminate any threats to GPS as soon as possible. There is a need for methods of rejecting an interfering signal and methods of determining the direction of the interfering signal, and if possible, determining the location of its source.
The most common methods of interference suppression/rejection are beam steering, null steering, signal cancellation, polarization filtering, frequency incision, tapped-delay lines, and adaptive signal processing. With the exception of polarization filtering, frequency incision, and possibly adaptive signal processing methods, most of these techniques require multiple RF channels and antenna elements or phased arrays to successfully eliminate interfering signals. A good description of interference mitigation techniques can be found in Ghose [1996].
The angle-of-arrival (AOA) of a signal can be obtained through either monopulse or sequential direction-finding systems that are either active or passive, or through the use of interferometric systems. A direction-finding system is basically comprised of one or more antennas or antenna elements and a receiver such that the azimuth and/or the elevation angle of an incoming signal can be determined. Direction-finding systems use either scalar or vector processing to determine the AOA of a signal. Scalar systems work with either the amplitude or phase of a signal while vector systems work with both amplitude and phase. The receiver of a DF system can be either monopulse or sequential and may have one or more radio frequency (RF) channels. Single-channel systems either use a rotating antenna element or sequentially switch between two or more antenna outputs. In general, however, AOA information is obtained by comparing the amplitude and/or the phase of two or more RF channels. Amplitude-comparison systems measure the relative amplitude of two or more channels to determine the AOA while phase-comparison systems measure the relative phase between channels. Hybrid systems that measure both relative amplitude and phase are referred to as amplitude-phase-comparison systems. The comparison takes place either simultaneously (monopulse), or sequentially. Monopulse systems are more robust because they eliminate the effects of emitter phase and amplitude variations as a function of time. Depending on the application, DF systems measure either the elevation (θ), or azimuth (φ) angle-of-arrival, or both. A detailed analysis of DF systems can be found in Kennedy et al. [1984] and Lipsky [1987].
The location of the emitter is generally determined by triangulation of simultaneous (or near-simultaneous) AOA measurements from multiple DF systems that are spatially diverse, or through multiple AOA measurements from a moving DF system. In order to determine the location of an emitter it is also necessary to know the position of the DF sensor for each AOA measurement. A DF system can also be used as part of a homing system that is designed to guide a vehicle toward an emitter.
Phased-array systems are capable of providing both interference rejection/suppression and the AOA of the interfering signals. Interference suppression in conventional adaptive phased-array systems is achieved by summing the weighted outputs from two or more antenna elements. A processor determines a complex weight or set of weights for each output signal. If the weights are chosen correctly, the effective power of the interference in the final output will be significantly reduced and the desired signal strength will be enhanced. This approach to interference mitigation is performed solely within an electronic package that has two or more antenna input ports. Each such port is connected to an antenna element via an RF (radio or carrier frequency) transmission line of some type. The antenna elements are designed to have coverage that is as broad as possible but are offset from each other in position and/or orientation. These offsets have to be large enough so that there are sufficient signal phase differences among the individual element outputs. The processor uses these phase differences to advantage in determining the appropriate weights. For adequate spatial filtering, element separations ranging from 0.3 to 0.5 carrier wavelengths are required. The elements are typically passive (have fixed properties) and all the interference mitigation is provided within the system electronics package. Thus, the RF or front-end of the system is not affected by the interference-mitigating functions of the antenna system.
Phased-array antenna systems can be very effective in mitigating the impact of one or several interfering sources. Moreover, the complex weights of a phased-array antenna system can also be used to determine the AOA of one or more interfering signals. However, they also have drawbacks. The two most significant ones are:                (1) The outputs of multiple antenna elements must be handled simultaneously. This means multiple matching networks, filters, and down-converters and possibly multiple LNAs at the front-end. For some applications, the system will also require multiple AD converters.        (2) The required total antenna aperture may be unacceptably large for many applications.        
Hence, there is a need for a low-cost, compact antenna system that is capable of providing good interference rejection and, if desired, the AOA of the interfering signal.